You can't exactly create a separate library, but you can assign those books a tag -- say, [Aunt Susie] -- and then use a restriction to simulate separate libraries. Just save a search for that tag, and then use that saved search as a restriction, and you've effectively got an Aunt-Susie-specific library. Save a negated version of that search (everything but the [Aunt Susie] tag) and you'll have your library without her books.